1. Field of the Invention
The present invention relates generally to reinforcing the welds between riser pipes and riser braces in jet pump assemblies of boiling water reactors. In particular, the present invention relates to apparatus and methods for mechanically reinforcing the welds between riser pipes and riser braces by clamping the riser pipes to the riser braces.
2. Discussion of the Related Art
A typical boiling water reactor includes a reactor vessel, a core shroud disposed within the reactor vessel, and a core comprising a plurality of fuel assemblies disposed within the core shroud. An annular region or space between the core shroud and the reactor vessel is known as the downcomer annulus. Feedwater enters the reactor vessel via a feedwater inlet and is distributed circumferentially within the reactor vessel by a feedwater sparger. The feedwater mixes with other water coming from steam separators of the reactor and flows downwardly from the feedwater sparger through the downcomer annulus to enter a lower plenum of the core. Boiling is produced in the core, creating a mixture of water and steam that enters an upper plenum of the core and is directed into steam plenum heads or stand pipes. The mixture of water and steam flows through the stand pipes and enters a plurality of steam separators. The separated liquid water mixes with incoming feedwater and flows down the downcomer annulus to the core. The steam, on the other hand, passes through a dryer and is withdrawn from the reactor vessel via a steam outlet.
Boiling water reactors typically include a coolant recirculation system providing forced convection flow through the core. A portion of the water flowing through the downcomer annulus is withdrawn from the reactor vessel via a recirculation water outlet and is fed under pressure into a plurality of jet pump assemblies distributed about the core shroud within the downcomer annulus. The jet pump assemblies produce a forced convection flow through the core, thusly providing the required reactor core water flow. Boiling water reactors typically include between six and twelve jet pump assemblies with most boiling water reactors having ten jet pump assemblies.
Each jet pump assembly comprises a transition piece, a riser pipe extending downwardly from the transition piece to an elbow connecting the riser pipe to a recirculation inlet nozzle along the reactor vessel wall, and a pair of inlet mixers extending downwardly from the transition piece to a pair of diffusers mounted over holes in a pump deck. The pump deck connects a bottom portion of the shroud with the reactor vessel. The riser pipe is typically cylindrical and tubular and is normally oriented vertically within the downcomer annulus in parallel relation to the wall of the shroud. The riser elbow is tubular and bends outwardly toward the recirculation inlet nozzle. Normally, the outer end of the elbow is connected with a thermal sleeve in the recirculation inlet nozzle. The transition piece extends in opposite lateral directions at the top of the riser pipe to connect with the inlet mixers on opposite sides of the riser pipe. The inlet mixers are oriented vertically in the downcomer annulus in parallel relation to the riser pipe, with lateral support for the inlet mixers provided by respective restrainer brackets attached between the inlet mixers and the riser pipe.
The riser pipe is supported and stabilized within the reactor vessel by a riser brace attached to the riser pipe and to an attachment wall, the attachment wall typically being the reactor vessel wall. Commonly, the riser brace is attached to the riser pipe and to the attachment wall by being welded to the riser pipe and to the attachment wall. The riser brace ordinarily comprises a yoke and side members extending respectively from opposite ends of the yoke in spaced parallel relation. Typically, the yoke has an inwardly curved surface between the side members complementary to the outer curvature of the exterior surface of the riser pipe. The riser brace is disposed in the downcomer annulus with the riser pipe disposed between the side members. The riser brace is normally attached to the riser pipe via a weld between the inwardly curved surface and the exterior surface of the riser pipe. The riser brace is welded to the riser pipe with the side members transverse to the riser pipe and extending from the yoke to respective ends of the side members attached to the attachment wall. The ends of the side members are normally attached to the attachment wall by welding, and the ends of the side members may be welded to intermediary structure, such as braces, blocks or pads, with the intermediary structure being in turn welded to the attachment wall. Typically, each side member of the riser brace comprises an upper leg and a lower leg disposed beneath the upper leg in spaced parallel relation therewith.
The riser brace provides lateral and radial support to the riser pipe. In addition, the riser brace is designed to accommodate the differential thermal expansion resulting from reactor start-up and heat-up, and to accommodate the flow-induced vibration incumbent in the reactor water circulation system due to reactor recirculation pumps. It is important that the natural frequency of the riser brace be greater than the vane passing frequency of the recirculation pumps at any pump speed to avoid the riser brace going into resonance. If the vane passing frequency of the recirculation pumps equals or exceeds the natural frequency of the riser brace, the riser brace will go into resonance and potentially become unstable to the detriment of the jet pump assembly.
lntergranular stress corrosion cracking (IGSCC) resulting from corrosion, radiation and/or stress may occur in the welds between the riser braces and the riser pipes of jet pump assemblies of boiling water reactors. Cracks initiated by IGSCC or other causes in the welds between the riser braces and the riser pipes may grow in size and reach critical sizes for mechanical fatigue resulting from the vane passing frequencies of the recirculation pumps exceeding the excitation frequency of the riser braces.
A clamp apparatus for stiffening a riser brace of a jet pump assembly is disclosed in U.S. Pat. No. 6,647,083 B1 to Jensen. The clamp apparatus is applied to the side members of the riser brace to shorten portions of the side members subject to vibration. The clamp apparatus does not attach to the riser pipe and does not augment the welded connection between the riser brace and the riser pipe.
Various clamps used in jet pump assemblies of boiling water reactors are represented by U.S. Pat. Nos. 6,463,114 B1 to Wivagg, 6,490,331 B2 to Erbes, 6,450,774 B1 to Erbes et al, 6,086,120 and 6,053,652 to Deaver et al, and 6,108,391 to Deaver, and by Japanese Patent Publication No. 11326586A.
The Wivagg patent discloses a clamp used in conjunction with a jacking device to restrain the existing jack screws that are welded about the peripheries of the inlet mixers to provide lateral restraint for the inlet mixers within the restrainer brackets.
The Erbes patent relates to a spring clamp for providing a tight fit between an inlet mixer and a restrainer bracket.
The Erbes et al patent discloses a clamp for being installed on a slip joint coupling an inlet mixer to a diffuser. The clamp is used to squeeze the diffuser to impart an oval deformation to the diffuser.
The Deaver et al patents ('120 and '652) disclose a clamp apparatus for supporting the lower portion of a riser of a jet pump assembly. The clamp apparatus comprises an elbow clamp, a riser clamp and a bridge coupling the elbow and riser clamps. The riser clamp includes a pair of legs for being disposed on opposite sides of the riser pipe and a back portion rigidly connecting the legs in fixed relation.
The Deaver ('391) patent and the Japanese Patent Publication relate to a clamp having upper and lower clamp elements receiving the outer end of a riser elbow therebetween.
Clamp apparatus involved with replacing cracked core spray supply piping in a boiling water reactor are represented by U.S. Pat. Nos. 6,195,892 B1 and 5,839,192 to Weems et al.
U.S. Pat. No. 1,400,801 to Cohen discloses a clamp having opposed jaws movable toward and away from each other along a longitudinal axis of movement. Arms of the jaws extend perpendicular to the axis of movement at a fixed angle.
It can be seen from the above that there is a need for an apparatus and method for mechanically reinforcing the weld between a riser pipe and a riser brace of a jet pump assembly. There is a particular need for a mechanical clamp assembly for mitigating or repairing a cracked weld between a riser brace and riser pipe of a jet pump assembly to maintain structural integrity of the jet pump assembly and to avoid excessive vibration of the riser brace. There is an additional need for a clamp assembly to provide redundant structural support to the weld between a riser pipe and riser brace of a jet pump assembly. Another need exists for a clamp assembly capable of attaching to an existing riser pipe and riser brace of a jet pump assembly while closely matching the footprint of the riser brace. There is also a need for a clamp assembly for applying a compressive load between a riser pipe and the yoke of a riser brace in a jet pump assembly. A need further exists for a clamp assembly providing an alternate load path for loads from a riser pipe to a riser brace and ultimately to the attachment wall to which the riser brace is attached in a boiling water reactor. An additional need exists for a mechanically reinforced weld between a riser pipe and a riser brace of a jet pump assembly wherein the riser brace is clamped to the riser pipe. A need also exists for a method of mechanically reinforcing the weld between a riser pipe and riser brace of a jet pump assembly.